[Aavso-photometry] Differential photometry for dummies

[arne]

R. Brian Potter wrote:
> So I totally understand that I should not combine images, but rather analyze
> the individual images after subtracting a master dark created from at least
> 7+ images.
> 
> My questions remaining are
> 1) How do I measure signal-to-noise (specifically in CCDSoft if someone is
> doing this)
> 2) How do I provide an error estimate for my results? (Std deviation on
> multiple measurements, 1/SNR, ???)
> 3) The CCD Observing Manual states that the observed K-C should be equal to
> the known K-C.  Using the 9.8 mag star in the field for R Uma as my comp
> star gives a value of 8.04 for my check star, vs a known value of 8.2.  How
> close is close enough? If its not exactly equal to the known value, do I
> make an adjustment to the magnitude of my target star to account for this?
> 
> Sorry for all the questions, but I want to make sure I do things correctly
> for my first CCD submission.
> 
Error estimates are not necessary for your first submission; wait
until you have the other techniques under your belt first.  For
example, without flatfielding, you have some very potentially large
systematic errors (such as vignetting); trying to figure out
signal/noise and all the rest is irrelevant until you have the
observing methodology correct.

Also, don't worry too much if your K-C doesn't match a chart's K-C.
For this particular example, the true photometry for the two stars is:

ID    Tycho2         RA  (J2000) DEC         B      V   sptype
82 TYC 4385-1737  10:42:28.74 +68:36:14.0   8.512 8.238 A3
98 TYC 4385-1176  10:42:41.55 +68:31:56.8  10.906 9.788 K0

so close to the chart values; however, note that the chart values
are always rounded to the nearest tenth (so an immediate built-in
error), and often are photographic or visual estimates and not
true precision photometry (such sequences typically have larger
error the fainter you go).  Report using your 9.8 comp star and
don't worry at this stage about the K-C difference as there are
lots of contributing factors (flatfielding, color difference between
the stars, etc.) that you have not yet corrected for.

Your next step along the path to photometric precision is flatfielding,
and I suggest you look at that step before worrying about much of
anything else.
Arne